The present invention relates to a method and apparatus for detecting the motion of image in a bandwidth-compressed television transmission signal which does not include the interframe aliased portion in the lower frequency region, and particularly to a method and apparatus for the image motion detection intended for the decoder in the receiver for receiving the compressed television transmission signal.
One of methods for transmitting a television signal, particularly a high definition television signal, by bandwidth compression is the multiple subsampling transmission system which implements offset subsampling between frames and between fields. An example of this system called MUSE (Multiple Sub-Nyquist Sampling Encoding) is practiced currently and it accomplishes effective bandwidth compression. The MUSE system is described in detail in the technical study report of The Institute of Electronics and Communication Engineers of Japan, Graphic Engineering IE 84-72. In the above-mentioned transmission system, subsampling cycles in two frames and therefore the receiver cannot use a difference signal for immediately adjacent frames (because of absent partner of comparison) and has to use a difference signal between next adjacent frames in detecting the motion of image, resulting in an inaccurate motion detection. With the intention of overcoming the above problem and yet simplifying the receiver and improving the picture quality, the applicant of the present invention has proposed a multiple subsampling transmission system which implements the motion detection using the difference signal for immediately adjacent frames (refer to Japanese Patent Unexamined Publication No. 264889/86 (JP-A-61-264889)).
The above-mentioned proposal resides in the multiple subsampling transmission system which compresses the transmission bandwidth by offset subsampling between fields and between frames, wherein the television picture signal is initially subjected to interfield offset subsampling, and the resulting signal is processed with a low-pass filter having a cut-off frequency half the sampling frequency of interfield offset subsampling, and then the filtered signal is subjected to frequency conversion and interframe offset subsampling at a frequency lower than twice the low-pass filter cut-off frequency. The produced transmission signal does not include the interframe aliased portion in the lower frequency region, enabling a low-frequency component signal extracted in the receiver to produce the interframe difference signal (difference signal for immediately adjacent frames) as a motion signal, whereby accurate motion detection is accomplished.
However, even the above transmission system is imperfect in motion detection in some cases, particularly when a small portion of picture has moved, and with the intention of improving the accuracy of motion detection, the applicant of the present invention has proposed "method of detecting the motion of image in the multiple subsampled transmission signal" in Japanese Patent Unexamined Publication No. 172876/87 (JP-A-62-172876).
According to the motion detection system of the Japanese Patent Unexamined Publication No. 172816/87, the decoder of the multiple subsampling transmission system without including the interframe aliased portion in the lower frequency region (proposed in Japanese Patent Unexamined Publication No. 264889/86) operates to switch the band of the immediate-adjacent interframe difference detection signal for producing the motion detection signal to a region which includes the aliased portion or a region which does not include to depending on the result of detection for the next-adjacent interframe difference (a difference between next adjacent frames). The switching operation, in case the detected next-adjacent interframe difference is significant having a magnitude higher than the reference level, allows the band of the motion detecting immediate-adjacent interframe difference signal to be intermingled with a component higher than the cut-off frequency (e.g., 4 MHz) of the low-pass filter used in the decoder of the Japanese Patent Unexamined Publication No. 264889/86, namely the aliased portion, so as to achieve accurate motion detection for smaller portions, while ensuring an immediate-adjacent interframe difference signal without the aliased portion for the case of a fast movement of a larger object, thereby stabilizing the motion detection.
However, among three signals (the next-adjacent interframe difference signal, immediate-adjacent interframe difference narrow-band signal and immediate-adjacent interframe difference broad-band signal), when one (e.g., next-adjacent interframe difference signal) is used dominantly as the above-mentioned motion signal, the system becomes increasingly dependent on the dominating signal. For example, when small portions of picture move right and left due to the slow swing of the television camera or the like, the dominating next-adjacent interframe difference signal will be made valid or invalid alternately. If this phenomenon occurs at every frame, the pixelwise motion signal will become unstable, with its state changing in every frame. Consequently, the motion signal is switched to match with the unstable dominating signal, resulting likely in a deteriorated picture. Therefore, it is necessary to use the above-mentioned three signals appropriately as a motion signal, instead of using one signal dominantly which causes the motion signal to become unstable.